1. Field of the Invention
The present invention relates to a superconducting magnet apparatus and a method for magnetizing a superconductor and, more particularly, to an apparatus that causes a bulk high-temperature superconductor to capture a great magnetic field and makes it possible to use the superconductor as a magnet and a method for magnetizing the superconductor.
2. Description of the Related Art
Through structure control, some high-temperature superconductors formed from, for example, yttrium (Y)-system materials, have been developed that are able to capture great magnetic fields exceeding 1 T, which is impossible for permanent magnets to capture, at a liquid nitrogen temperature level. These superconductors are capable of capturing increased magnetic fields if they are cooled to lower temperatures. Moreover, since property improvements are expected due to developments in the field of materials, use of the superconductors as strong magnets is lately considered.
There are mainly two methods for magnetizing a bulk superconductor: a so-called FC (Field Cooling) method that cools a bulk superconductor to the superconduction transition temperature Tc of the superconductor or a lower temperature while applying a magnetic field to the superconductor; and a so-called ZFC (Zero Field Cooling) method that cools a bulk superconductor to its superconduction transition temperature or lower and then applies a magnetic field to it from the outside so that the magnetic field penetrates into the superconductor. In either method, it is necessary to apply a magnetic field at least equal to a magnetic field that the superconductor is desired to capture, to the superconductor at least once. Furthermore, it is necessary to maintain the temperature of the superconductor at a temperature equal to or lower than the temperature at the time of magnetization, in order to maintain the magnetic field captured by the superconductor.
The FC magnetization method has normally been employed to cause a high-temperature superconductor to capture a magnetic field for the purpose of, for example, evaluating the characteristics of the superconductor. For example, a technology disclosed in Japanese Patent Laid-Open No. Hei 7-111213 uses the FC method to cause a superconductor to capture a magnetic field, and produces a magnet by combining the superconductor and a coil.
In the ZFC magnetization method, on the other hand, after a superconductor is cooled, an external magnetic field is slowly applied to the superconductor and then slowly reduced to zero. Since the superconductor has already been cooled to the superconducting state at the time of application of the external magnetic field, a certain amount of the external magnetic field applied is expelled. Therefore, the ZFC method requires application of a greater magnetic field than the FC method. This is part of the reason why if a steady magnetic field is to be used for magnetization, the FC method, not the ZFC method, is normally employed for practical purposes.
Besides the foregoing methods, which simply turns a bulk superconductor directly into a magnet, another magnetization method is disclosed in Japanese Patent Laid-Open No. Hei 5-175034. In this method, a bulk superconductor is formed into the shape of a coil, and the coil-shaped superconductor is magnetized by supplying electricity to the superconductor.
The conventional FC method requires that a steady magnetic field be applied to a superconductor while the superconductor is being cooled. However, the steady magnetic field can be produced only in a small magnitude if a simply-constructed magnetic field generator is employed. Therefore, as long as a simple generator is employed in the FC method, it is normally impossible to cause a superconductor to capture a magnetic field that considerably exceeds the magnetic field of a normal permanent magnet.
A Nbxe2x80x94Ti superconducting coil can be used in the FC method to produce a great steady magnetic field to be applied to a superconductor. However, since the Nbxe2x80x94Ti superconducting coil needs to be cooled to a very low temperature, the entire apparatus for performing this method normally needs to be increased in size and complexity in order to cause the superconductor to capture a great magnetic field.
Furthermore, since the superconductor must be cooled while being subjected to a magnetic field, the FC method requires a long time for magnetization. In addition, after magnetization, the superconductor must be continually cooled even when installed for use, thus considerably limiting the location of use. Therefore, the FC method is not suitable for the purpose of using a superconductor as a strong magnet disposed inside an apparatus or thy like.
If the ZFC method uses a steady magnetic field, the method suffers from problems similar tn those of the FC method. Moreover, since the ZFC method requires a greater applied magnetic field than the FC method, the problems become more remarkable in the ZFC method.
In a method wherein a bulk superconductor is formed into the shape of a coil as disclosed in Japanese Patent Laid-Open No. Hei 5-175034, the working on the superconductor becomes considerably complicated and, if a ceramic superconductor is used, the working becomes very difficult and costly. Furthermore, deterioration of the material during the working is likely, thereby making it difficult to produce a superconductor having stable properties.
According to the foregoing conventional methods, even though bulk superconductors with good properties are available, it is difficult to use such bulk superconductors as magnets that produce great magnetic fields in various appliances and machines.
Japanese Patent Laid-Open No. Hei 6-168823 describes a method that applies pulse-like magnetic fields to a superconductor instead of a steady magnetic field. This method is very useful to magnetize a superconductor using a simple coil device.
The present invention is directed to an improvement of a superconducting magnet apparatus for pulsed magnetization and a pulsed magnetization method that are described in Japanese Patent Laid-Open No. Hei 6-168823. It is an object of the present invention to provide simple apparatus and method for causing a bulk superconductor to capture a conventionally unachievable high magnetic field, without performing machining or another working process on the superconductor, thereby making it possible to use a superconductor as a magnet in various appliances for various applications.
To achieve the aforementioned object of the invention, the present inventors have attempted to improve the pulsed magnetization method. It is conventionally considered that in the pulsed magnetization method, the space between a superconductor and a magnetizing coil needs to be minimized because when a magnetic field is applied to magnetize a superconductor that has been cooled without being magnetized, the superconductor exhibits a characteristic of expelling the entering magnetic field. However, it is desirable that the magnetizing coil and the superconductor be more freely arranged in order to use the superconductor as a magnet in various apparatuses. Accordingly, in view of designing a magnet apparatus in various arrangements with an increased freedom, the present inventors considered and examined various conditions, such as the arrangement of a superconductor and a magnetizing coil, the magnitude of pulsed magnetic fields, duration of application of pulsed magnetic fields, the manner of applying pulsed magnetic fields and the like.
According an aspect of the present invention, there is provided a method for magnetizing a superconductor which method includes cooling a superconductor, and magnetizing the superconductor by supplying a magnetizing coil with a pulsed current whose peak value is controlled beforehand, and by causing a magnetic field produced by the magnetizing coil to penetrate into the superconductor and causing the superconductor to capture a magnetic field.
The magnetic field captured by a superconductor is dependent on the critical current density Jc of the superconductor and the configuration of the superconductor, and there exists an upper limit (maximum captured magnetic field) of the magnetic field captured by the superconductor under certain conditions. If a peak value of a pulsed current to be supplied to the magnetizing coil is small, the magnetic field that penetrates into the superconductor becomes also small. In such a case, an insufficient captured magnetic field may result although a maximum captured magnetic field is desired. However, if a peak value of a pulsed current to be supplied to the magnetizing coil is controlled beforehand, the magnetic field that penetrates into the superconductor is correspondingly controlled. Therefore, it becomes possible for the superconductor to capture a magnetic field comparable to a desired captured magnetic field.
According to another aspect of the present invention, there is provided a method for magnetizing a superconductor which method includes cooling a superconductor, and magnetizing the superconductor by energizing a magnetizing coil that is disposed facing at least one of two opposite sides of the superconductor in a direction in which the superconductor is to be magnetized, and by causing a magnetic field produced by the magnetizing coil to penetrate into the superconductor and causing the superconductor to capture a magnetic field.
Since the magnetizing coil faces at least one of two opposite sides of the superconductor where magnetization surfaces exit, local magnetization of the superconductor can be achieved by disposing the magnetizing coil facing only a desired magnetization surface, and then performing pulsed magnetization. If uniform magnetization of the entire superconductor is desired, the magnetizing coil is disposed facing the magnetization surfaces of the entire superconductor to perform pulsed magnetization. Thus, this method is able to perform pulsed magnetization locally or entirely on the superconductor.
According to still another aspect of the present invention, there is provided a superconducting magnet apparatus having a superconductor disposed in an insulating container, a refrigerator provided with a cold head that thermally contacts the superconductor and cools the superconductor, and a magnetizing coil that applies a pulsed magnetic field to the superconductor. An energization device is provided for energizing the magnetizing coil by a pulsed current.
Since the superconductor is cooled by the refrigerator provided with the cold head, the superconducting magnet apparatus is able to set the temperature of the superconductor to be reached by cooling to any desired temperature, unlike an apparatus that uses a coolant, such as liquid nitrogen or the like, to cool a superconductor. Normally, the properties of superconductors are affected by the temperature of the superconductors. Therefore, the setting of the superconductor temperature to any temperature makes it possible to produce superconducting magnets having various properties.
According to a further aspect of the present invention, there is provided a superconducting magnet apparatus having a superconductor disposed in an insulating container, a cooler device for cooling the superconductor, and a magnetizing coil that applies a pulsed magnetic field to the superconductor. The magnetic coil is disposed outside the insulating container. Energization device is provided for energizing the magnetizing coil by a pulsed current.
Since the magnetizing coil for applying a pulsed magnetic field to superconductor is disposed outside the insulating container containing the superconductor, the superconductor is not affected by heat generated from the magnetizing coil during magnetization performed by supplying the pulsed current to the coil; that is, a rise of the temperature of the superconductor caused by an external factor is avoided. Therefore, it becomes possible to perform further stable pulsed magnetization leading to stable properties of the superconductor. Furthermore, the insulating container containing a superconducting magnet; that is, the superconductor that has captured a magnetic field can easily be separated from the magnetizing coil, a magnetizing power source and the like, so the portability of the superconducting magnet is improved.
According to a still further aspect of the present invention, there is provided a superconducting magnet apparatus having a superconductor disposed in an insulating container, a cooler device for cooling the superconductor, and a magnetizing coil that applies a pulsed magnetic field to the superconductor. A heater device is provided for heating the superconductor.
Since the heater device for heating the superconductor is provided, the apparatus is able to achieve any desired temperature distribution in the superconductor. By performing pulsed magnetization a plurality of times with various temperature distributions in the superconductor, the superconductor can be caused to capture a maximum possible magnetic field.
According to a yet further aspect of the present invention, there is provided a superconducting magnet apparatus having a superconductor disposed in an insulating container, a cooler device for cooling the superconductor, and a magnetizing coil that applies a pulsed magnetic field to the superconductor. The magnetizing coil is disposed facing at least one of two opposite sides of the superconductor in a direction in which the superconductor is to be magnetized.
Since the magnetizing coil faces at least one of two opposite sides of the superconductor where magnetization surfaces exit, local magnetization of the superconductor can be achieved by disposing the magnetizing coil facing only a desired magnetization surface, and then performing pulsed magnetization. If uniform magnetization of the entire superconductor is desired, the magnetizing coil is disposed facing the magnetization surfaces of the entire superconductor to perform pulsed magnetization. Thus, this apparatus is able to perform pulsed magnetization locally or entirely on the superconductor.